HARRY Potter author Joanne Rowling revealed yesterday she almost had a nervous breakdown writing her new blockbuster.

The 34-year-old mum said she was forced to rework the entire storyline after discovering a basic flaw in the make-believe magic world she spent five years creating.

Joanne said: "For the first time ever I lost my careful plot - which I'd had since 1994, I think.

"I really should have gone through it with a fine toothcomb before I started writing and I didn't.

"I had a false sense of security because all my other plans had held up so well."

Harry Potter And The Goblet Of Fire, the fourth tale of the schoolboy wizard's adventures, has been shattering publishing records since its release at midnight on Friday.

It took £5million in advance orders, boosting the fortune of the Edinburgh writer to £15million plus.

But Joanne, who was on the dole five years ago, admitted yesterday it was the hardest book she had ever written.

The 636-page novel took twice as long as any of her three previous classics.

"Book Four nearly caused me a nervous breakdown," she said. "I sailed straight into the writing of it, having just finished Azkaban.

"I had written what I thought at the time was half the book - it turns out now to have been about a third.

"And I realised there was this big hole in the middle of the plot and I had to go back and unpick and redo.

"That's part of the reason it's longer than I thought it was going to be."

Joanne said she would write - in longhand - while her six-year-old daughter Jessica was at school, then stop when she came home.

"Sometimes I don't down tools for the day, sometimes I go back to it in the evening," she said.

"But on Book Four I was working 10-hour days."

Joanne admitted she suffered writer's block during her second book, Chamber Of Secrets.

She said: "I think it was panic because I got this big burst of publicity for Philosopher's Stone and I froze. I thought Chamber of Secrets would never be as good.

"But the writing itself has never stopped being completely joyful."

Joanne unwound while writing her latest book by watching The Simpsons on TV with Jessica.

She said the cartoon series was similar to her work because it was based on certain unbreakable rules.

"The most important thing to decide when you're creating a fantasy world is what the characters CAN'T do - like The Simpsons, which is a work of genius," Joanne said. "You can tell that they've structured it in such a way that they're never at a loss for what their characters can and can't do.

"That's why they're so believable even though they're little yellow people. Lisa is my favourite. I love Marge as well."

Joanne also revealed she is a Spurs football fan. "An ex-boyfriend of mine was a Chelsea fan and our relationship completely followed Chelsea's fortunes," she laughed.

"They got relegated and we split up - then they had a fantastic season and got promoted and we got back together again.

"I try and think that was coincidence but I fear not."

Joanne plans to write three more Harry Potter books.

That will delight her young fans, who turned out in force to meet her yesterday on her Hogwart Express steam train tour of Britain to promote the new novel.

She rattled off another 600 book signings amid chaotic scenes at Newcastle upon Tyne before steaming up to Edinburgh for more.

Erin Trant, 12, from Lockerbie, said after meeting Joanne: "I bought my copy on Saturday morning and I have spent all weekend reading it. I think it is by far the best book she has written."

Middlesbrough friends Emma Carr and Emily Robson, both 10, were dressed as Hermione, the bossy witch in the Harry Potter books.

Emma said: "The latest one is fantastic. I would give it 10 out of 10."